The suspend-and-resume facility may be provided on an individual subchannel
basis for nonshared subchannels. That
is, if suspend-and-resume facilities are
provided by the model, they are provided
for one or more nonshared subchannels of
one or more multiplexer channels. The
suspend-and-resume facility is not
provided for shared subchannels, includ­
ing the subchannel of a selector
channel.
When channel-program execution is initi­
ated via SIO or SIOF executed while the
block-multiplexing-control bit (bit 0 of
control register 0) is zero, the
suspend-and-resume facility, if provided
for the subchannel, mayor may not be
operable. When the facility is not
operable, detection of the S flag in a
CCW causes the channel to recognize the
program-check condition and terminate
the operation.
Suspension occurs when a new CCW takes
control that has a valid S flag. The
command field of this CCW is not sent to
the I/O device, and the device is
signaled that the chain of commands is terminated. The CCW containing the S
flag must be a valid CCW since all
normal CCW checking is performed. A
subsequent RESUME I/O instruction
informs the channel that the suspend CCW
may have been modified and that the
channel must refetch the CCW and examine
the current settings of the flags. The
channel never executes a CCW with the S flag, regardless of the number of RIO instructions executed.
If the CCW containing the S flag also
contains the PCI flag, an interruption
condition is generated and made pending at the subchannel or device after
channel-program execution is suspended.
The PCI is presented to the program when
it is allowed, regardless of whether the
channel-program execution is still
suspended or not. The suspend function,
when used in conjunction with PCI, serves as a mechanism for alerting the
program to the occurrence of a suspen­
sion at the subchannel.
When the first CCW of an I/O operation has the suspend flag validly set to one,
the operation is suspended prior to initiating the operation at the device. When this occurs, condition code 0 is
set for START I/O. Thus, when suspen­
sion occurs on the first CCW, a START
I/O initiating an immediate operation
for which command chaining is not speci­ fied in the CCW causes a condition code 0, rather than a condition code 1, to be
set.
Programming 1. In certain situations, normal
resumption of a suspended channel
program may not be desired. Normal
termination of the suspended
program may be by:
a. Executing HALT DEVICE\addressed
to the device. \
b. Modifying the CCWs in storage
such that when channel-program
execution is resumed, the first
command issued to the device is a control command with modifier
bits of all zeros
(no-operation) and with no
chain-command flag specified,
and then issuing RESUME I/O. 2. If the command code of a CCW that
caused suspension of channel­
program execution is replaced by
the transfer-in-channel command
code (X8 hex) prior to executing
RIO, the S flag need not be removed
from the CCW because bits 32-63 of
the CCW are ignored when the
command is transfer in channel
(TIC).
3. In some models, the suspend-and­
resume facility is operable for a
channel-program execution that is
initiated on a block-multiplexer
channel while the block­
multiplexing-control bit (bit 0 of
control register 0) is zero. In
these models, channel-program
execution occurs with multiplexing
inhibited until the channel-program
execution is suspended. When
suspension occurs, the effect on
the channel is the same as if block
multiplexing had occurred. That
is, the device is disconnected from
the channel at the end of a block,
and the subchannel remains in the
working state. When this happens,
the channel becomes available for a
new SIO function for some other
device.
When the suspended channel-program
execution is subsequently resumed,
it is executed as if a new
channel-program execution were initiated via an SIOF function with
the block-multiplexing-control bit
set to one. That is, block multi­
plexing is no longer inhibited
after the channel-program execution
is resumed.
Chapter 13. Input/Output Operations 13-47

COMMANDS The figure "Channel-Command Codes" lists
the command codes for the seven valid
commands and indicates which flags are defined for each command. The flags are ignored for all commands for which they
are not defined.
Name Code Wri te MMMM MMOI CD CC SLI
Read MMMM MM10 CD CC SLI Read backward MMMM 1100 CD CC SLI Control MMMM MM11 CD CC SLI Sense MMMM 0100 CD CC SlI Sense 10 1110 0100 CD CC SLI Transfer in channel XXXX 1000 EXj2lanation: CD Chain data CC Chain command SLI Suppress length indication
SKIP Skip
pcr Program-controlled interruption
IDA Indirect data addressing
M Modifier bit S Suspend
X Ignored Channel-Command Codes 13-48 System/370 Principles of Operation
Flags PCI IDA S SKIP PCI IDA S SKIP PCI IDA S PCI IDA S SKIP PCI IDA S SKIP PCI IDA S